Discovery of critical phenomena in a quantum spin liquid

(Nanowerk News) For the first time, a NIMS research group led by Takayuki Isono (postdoctoral researcher) and Shinya Uji (Deputy Director-General of the Research Center for Functional Materials), and the University of Tokyo research group led by Professor Kazushi Kanoda (School of Engineering), jointly observed quantum critical behavior of the magnetic susceptibility for an organic material with a triangular lattice, when the material was in a quantum spin-liquid state at very low temperatures (Nature Communications, "Quantum criticality in an organic spin-liquid insulator κ-(BEDT-TTF)2Cu2(CN)3").

(Left) Scaling plot of magnetic susceptibility. Vertical axis represents scaled magnetic susceptibility, and different colors on the curve stand for different conditions under which measurements were taken. As you can see, magnetic susceptibilities exhibit a universal scaling function (dotted curve) across wide ranges of temperatures and magnetic fields. (Right) Magnetic phase diagram. The red area (quantum critical regime) represents critical behavior of magnetic susceptibility near a quantum critical point at absolute zero temperature and zero magnetic field. Existing theories cannot explain the critical exponents that were obtained. (click on image to enlarge)

In general, atomic and molecular arrangements of materials become more stable (ordered state) as they get colder, like water turning to ice as it gets colder. Quantum spin liquid is an exception to this law, and its electron spin states remain disorderly and unstable (liquid-like state) at very low temperatures.

A quantum spin-liquid state had been discovered in some materials, but it is still unknown how the liquid state is maintained at very low temperatures.

To understand the nature of quantum spin liquids, it is necessary to understand the universal properties of materials, which do not depend on their microscopic details (e.g., constituent elements).

In this research, the joint group grew a high-quality single crystal of an organic spin-liquid material, κ-(BEDT-TTF)2Cu2(CN)3, and took precise measurements of its magnetic susceptibility up to 17 tesla at extreme temperatures as low as 0.03 kelvin.

As a result, it was found that the materialís magnetic susceptibility tends to diverge with decreasing temperature, and the magnetic susceptibilities exhibit a universal scaling function across very wide ranges of temperatures and magnetic fields.

These results indicate that this material exhibits critical spin-liquid states near a quantum critical point at zero magnetic field. Moreover, the group successfully estimated critical exponents (universal quantities) that can be determined only by basic properties of the material, which do not depend on its microscopic details.

The critical exponents determined in this study will serve as powerful indices for sorting out theoretical models related to this mysterious liquid state. Existing theories did not explain the exponents we obtained experimentally.

Based on these exponent values, we hope that progress will be made in theoretical modeling and detailed understanding of the mechanism that drives quantum spin liquid phenomena.

Also, the relationship of quantum spin liquids to high-temperature superconductivity (for example, of copper oxide materials) is of great interest. As such, an advancement in theoretical understanding of spin liquids may also lead to understanding of the high-temperature superconductivity mechanism.